This invention relates to the continuous casting of steel, and more particularly to submerged entry nozzles for use in delivering molten steel between the tundish and mold of a continuous casting machine.
Continuous casting is a steel making process that transforms liquid steel into semi-finished slabs, blooms, and billets that can be further processed into finished products. In its operation, liquid steel is supplied by ladle to a casting machine tundish and fed through a submerged entry nozzle, or “SEN,” to a casting machine mold. The mold may be an open-ended box-like structure that provides the cast section with its desired shape. The mold may have four copper surfaced steel plates that function as the mold walls. The walls may be position adjusted inward and outward to change the width and thickness of the cast section, to produce slabs that are from, for example, about 50 to 230 millimeters (mm) thick and about 610 mm to 1520 mm wide. Water jackets in the copper lining provide primary cooling to the liquid steel that comes in contact with the mold walls, causing it to solidify and form a shell. Oscillating and vertical displacement of the mold prevents the solidifying shell from sticking to the walls.
The shell and its liquid core form a strand that is withdrawn from the mold by casting machine drive rollers at a rate that is substantially equal to the rate of flow of the liquid steel into the mold. This provides the continuous casting process with an operational steady state condition. As the strand exits the mold it is subjected to water spray or water mist secondary cooling which prevents reheating of the surface of the strand by the heat of the molten core, until the strand has traveled its “metallurgical length,” at which point the core has solidified sufficiently that the strand can be cut to desired length on exit from the casting machine.
In the casting machine, the liquid metal is gravity fed from the tundish to the mold at a flow rate established by the bore size of the SEN. Different nozzle bore shapes and sizes may be selected depending on the section size and shape to be cast and the casting speed. The steel flow can be changed as necessary for the control of the casting operation. This may be done with a stopper rod that is fitted to the SEN inlet to restrict all or any part of the melt flow, or by a slide-gate that is drawn wholly or partially across the SEN inlet. The operation of the stopper rod or slide-gate may be performed manually by an operator, or automatically in response to a feedback signal from a level sensor in the mold.
The flow dynamics of the molten steel moving from the tundish to the mold can affect the quality of the continuous cast steel. A part of the casting process is the initial solidification of the liquid steel at the meniscus, which is the point at which the top of the solidifying steel shell meets the mold wall and the liquid steel of the mold bath. This is where the surface of the final cast product is created, and defects such as surface cracks can form if problems, such as too severe level fluctuations, occur in the liquid surface. To lessen this probability, oil or mold powder is added to the surface of the liquid steel in the mold. The mold powder produces a mold slag layer on the liquid surface which protects the liquid steel from the open air, provides it with thermal insulation, and also absorbs inclusions that are present in the liquid steel. Slag also flows into the gap between the mold wall and the shell to provide lubrication to the shell-to-copper interface.
Another factor related to the surface quality of the cast steel is the presence of turbulence and other transient phenomena in the flow of the molten steel from the SEN into the mold. The SEN delivers the molten steel through outlet ports in its distribution zone, which is submerged in the mold bath, below the mold slag line. Among the prior art nozzles that are commonly used are those in which the distribution zone has outlet ports positioned in opposite-side lateral passages at the bottom of the nozzle, discharging liquid steel in opposite lateral directions into the longer width dimension of the mold. Two outlet ports in each lateral passage provide the “double roll flow pattern” known in the art, in which each lateral passage discharge provides two flows. One moves upward through the mold bath and curls along the under surface of the meniscus and back toward the nozzle, and the other curls downward and also returns toward the nozzle.
The opposite side upward flows heat the meniscus to maintain its temperature at a level sufficient to melt the mold powder and provide proper lubrication to the casting. They also produce a standing wave profile at the liquid steel surface, which causes the mold powder slag layer to be thinner at the meniscus than around the nozzle body. It may be desired that the standing wave have a low amplitude, or at least a stable amplitude. Too high an amplitude standing wave may result from too high of a velocity of the upward flow. A varying amplitude, as may result from disrupted or intermittent flow velocities of the opposite side flows, can shear off droplets of mold slag or foreign particles trapped at the meniscus into the flow and entrain them in the liquid steel. The resulting inclusions can also generate surface defects and surface cracks in the finished steel.
Compact Strip Production (CSP), which is the casting of thin slabs which are about 50 mm to 100 mm (2 to 4 inches) thick, may use a SEN with a narrow substantially rectangular distribution zone. A funnel may also be fitted to the top of the mold to receive the SEN. With the CSP narrower dimensions, inclusion entrapment may result from nozzle-to-mold flow patterns having higher flow velocity. A SEN for use in CSP casting may be capable of maintaining stable steel flow velocities to satisfy CSP throughput but low enough to lessen entraining particles from the mold slag layer. The SEN may further provide flows that provide stable steel consistency, and that are substantially balanced at its lateral outlet ports.
The present disclosure is a submerged entry nozzle (SEN) with improved flow characteristics and production of slab cast steel. The submerged entry nozzle (SEN) is provided for use in a casting machine to deliver molten steel from a tundish to a casting mold comprising a housing having:
an inlet capable of receiving an incoming flow of molten steel from the tundish;
a distribution zone capable of delivering the molten steel into the casting mold; and
a main body having a bore capable of conducting molten steel therethrough from said inlet to said distribution zone, said bore having sectional geometries capable of alternately compressing and decompressing the molten steel flow in flow path zones to alternately increase and decrease the steel flow velocity with at least two flow path zones capable of compressing the molten steel flow, and to deliver the molten steel from said distribution zone into the mold with flow turbulence inhibited.
The distribution zone may comprise first and second lateral passages having secondary flows formed by a flow divider, the lateral passages having baffles adjacent passage outlets dividing the molten steel secondary flows into four molten steel discharge flows delivering the molten steel to the mold in divergent directions.
The flow divider may have a leading edge having a radius of curvature for dividing the molten steel primary flow into the lateral passages with lessened flow turbulence, where the radius may be a maximum 5 mm radius. The flow divider may comprise a vertical section with opposite sides thereof forming surface contours directing molten steel flow through the lateral passages. Alternately, the flow divider may comprise a vertical section with substantially straight sides directing two molten steel discharge flows substantially vertically downward.
The housing of the SEN may transition along the sectional geometries of the main body from a substantially circular geometry to a substantially rectangular geometry having opposing side walls and opposing front and back walls at the distribution zone, the opposing front and back walls converging from the substantially circular geometry to the distribution zone.
The opposing side walls may transition from the substantially circular geometry to the substantially rectangular geometry at the distribution zone in an incremental manner. The opposing side walls may be altered incrementally along the bore to provide the sectional geometries, and the opposing front and back wall may converge in a continuous linear taper from the substantially circular geometry to the distribution zone. The sectional geometries may include an upper compression zone and a lower compression zone, the upper compression zone providing from three percent to ten percent compression of the molten steel flowing therethrough and the lower compression zone providing from three percent to ten percent compression of the molten steel flowing therethrough.
At least part of the main body of the SEN adjacent to a slag line when installed in the mold may comprise zirconia graphite.
Also disclosed is a method of continuously casting steel slabs comprising the steps of:
assembling a casting mold capable of continuous casting of melt slabs;
assembling a tundish above the casting mold capable of containing molten steel to be cast and having an outlet capable of discharging the molten steel for the tundish; and
introducing molten steel into the casting mold from the outlet of the tundish through a submerged entry nozzle (SEN) comprising a housing having an inlet capable of receiving an incoming flow of molten steel from the tundish, a distribution zone capable of delivering the molten steel to the mold, and a main body having a bore capable of conducting molten steel therethrough from said inlet to said distribution zone, said bore having sectional geometries capable of alternately compressing and decompressing the molten steel flow in flow path zones to alternately increase and decrease the steel flow velocity with at least two flow path zones capable of compressing the molten steel flow, and to deliver the molten steel from said distribution zone into the mold with flow turbulence inhibited.
A continuous slab caster may comprise a casting mold capable of continuous casting of melt slabs; a tundish positioned above the casting mold capable of containing molten steel to be cast and having an outlet capable of discharging the molten steel from the tundish; and a submerged entry nozzle (SEN) capable of introducing molten steel into the casting mold from the outlet of the tundish, and comprising a housing having an inlet capable of receiving an incoming flow of molten steel from the tundish, a distribution zone capable of delivering the molten steel to the mold, and a main body having a bore capable of conducting molten steel therethrough from said inlet to said distribution zone, said bore having sectional geometries capable of alternately compressing and decompressing the molten steel flow in flow path zones to alternately increase and decrease the steel flow velocity with at least two flow path zones capable of compressing the molten steel flow, and to deliver the molten steel from said distribution zone into the mold with flow turbulence inhibited.